1. Field of the Invention
The invention relates to output drivers, and in particular to dual-operation drivers capable of outputting differential signals compliant with low voltage differential signaling (LVDS) specification or transmission minimized differential signaling (TMDS) specification under differential modes.
2. Description of the Related Art
Low Voltage Differential Signaling (LVDS) is a high-speed, low power interface used in most notebook computers to create a direct digital connection between the central processing unit (CPU) and LCD display. It provides very high line transmission rate, requires little power and generates low noise. LVDS technology, while optimized for short cable runs, fails when applied to transition LVDS technology to external desktop monitors.
FIG. 1A is a diagram of output driver 141 and a corresponding input unit 181 of a receiver (not shown). As shown, the LVDS output driver 141 is a current mode line driver, creating a differential voltage at the input unit 182 of the receiver 18 by current steering. For example, the current Iref flows to ground through the signal line 19, the terminal impedance 2RT about 100Ω and the signal line /19 by turning on switches A and the switches B off, and vice versa.
Transition Minimized Differential Signaling (TMDS) is an electrical standard used to transmit digital data to a display. The signal is optimized to reduce electromagnetic interference (EMI), which allows faster signal transfer with increased accuracy. The differential circuitry in TMDS allows complimentary limited amplitude signals to be transmitted over twisted pair wires rather than more expensive coaxial cable. The LVDS transmitter encodes and serially transmits a data stream over a TMDS link to a TMDS receiver. Video and sync information are serialized and sent over three sets of twisted pair wires, with an additional pair of wires transmitting a clock signal for timing.
As shown in FIG. 1B, the TMDS output driver 142 is a current mode line driver, generating a differential signal over lines 191 and /191 by controlling the switches D and /D. For example, when the switch D is turned on, the current Idr on line 191 pulls down the voltage at the node N1 of the input device 184 in a TMDS receiver 18′ through the terminal impedance RT of about 50Ω. The other line /191, which carries no current at this time, is maintained at Avcc (3.3V), thus a differential voltage swing is achieved.
Currently, separate LVDS and TMDS transmitters in different chips are required for transmitting LVDS signals and TMDS signals.